The present invention relates to electrical discharge machining (EDM) devices, and more particularly to heads and advancement mechanisms for EDM devices.
Electrical discharge machining is conventionally used to machine very hard, electrically conductive workpieces, and the like, that are difficult or impossible to machine by conventional methods. In normal applications of EDM, the workpiece is immersed in a bath of dielectric fluid, and a power supply is applied across the cutting tool and the workpiece. When the cutting electrode is brought sufficiently proximite to the workpiece, the dielectric fluid breaks down, and an electric discharge or spark is generated across the space or gap between the electrode and the workpiece. With the electrical discharge, a minute amount of material is removed from the workpiece.
It will be appreciated that in EDM processes, the spacing between the two electrodes, or the "spark gap", is critical. If the spark gap becomes too small, or actual physical contact is made, a short circuit occurs between the cutting electrode and the workpiece. Such a short circuit results in either the fusing of the cutting electrode to the workpiece, or a melting or embrittling of the effected area of the workpiece. A fused electrode is very difficult and time consuming to remove, and if sufficient damage occurs, the workpiece must be scrapped.
The problem of maintaining a proper spark gap is further complicated when an EDM device is used to tap holes in or through a workpiece. In such tapping operations the cutting electrode must be rotated as it is advanced into the workpiece. Heretofore, due to the difficulty of maintaining the proper spark gap, thread tapping EDM devices were often forced to rely upon hand advancement of the cutting tool, in order to properly control the machining process. However, an operator is not always capable of reacting quickly enough when the spark gap becomes too small in order to reverse the advancement of the cutting electrode. Further, hand advanced machines are relatively slow, and difficult to accurately control. Such hand advanced machines require an operator's constant attendance, which greatly increases the labor costs of machining the workpiece.
Some EDM devices have utilized automatic advancement of the thread tapping electrode. However, the advance mechanism for the electrode is relatively complex, and cannot be readily converted to cut different thread styles and sizes. The set-up time for such prior machines is therefore substantial. Further, multiple EDM devices are required in order to produce a full range of machining operations. Thread tapping EDM devices, that are designed for rotational advancement of the electrode, are not capable of being used for conventional EDM applications, in which the cutting electrode does not rotate. Similarly, devices designed for conventional EDM applications cannot be used for thread tapping.